One of the known post-translational modifications of proteins is the oxidation of methionine residues to methionine sulfoxide [Met(O)]. An enzyme, peptide methionine sulfoxide reductase (MsrA), which has been detected in virtually all organisms examined, catalyzes the reduction of free and protein-bound methionine sulfoxide residues to methionine. The yeast peptide methionine sulfoxide reductase (MsrA) was overexpressed in a Saccharomyces cerevisiae null mutant of msrA using a high copy plasmid harboring the msrA gene and its promoter. When exposed to either hydrogen peroxide, paraquat, or 2,2,azobis-(2-amidino-propane) dihydrochloride treatment, the MsrA overexpressed strain grew better, had lower free and protein-bound methionine sulfoxide, and had a better survival rate under these conditions in comparison with the msrA mutant and its parent strain. Substitution of methionine with methionine sulfoxide in a medium lacking hydrogen peroxide had a little effect on the growth pattern, which suggests that the oxidation of free methionine in the growth medium was not the main cause of growth inhibition of the msrA mutant. UVA radiation did not result in obvious differences in survival rates between the three strains. An enhanced effect in resistance to hydrogen peroxide treatment was shown in human T-lymphocytes cells (Molt-4), that were stably transfected with the bovine msrA, and were exposed to hydrogen peroxide. These results support the proposition that the msrA gene is involved in the resistance of yeast and mammalian cells to oxidative stress. Studies were carried out in order to learn more about the enzyme structure and function. The conserved motif of GCFWG in the N-terminus of most MsrAs was mutated by changing the amino acid sequence CFW, one amino acid at a time. All mutants lost their ability to reduce either free or protein bound Met(O) and DMSO with no apparent change in their secondary structure. Also, it was shown that MsrA could preferably reduce the S form of Met(O) and not the R form.